May 18 is World Aids Vaccine Day and as leaders from 193 countries gather in Geneva for next week’s World Health Assembly we would like to reinforce the urgent need to accelerate the Aids response and call for continued efforts to ensure the development and delivery of a vaccine to end the epidemic.

During the past 30 years, more than 60-million people have become infected with the human immuno­deficiency virus (HIV), about 30-million have died and more than 16-million children have lost one or both parents to Aids.

Two-thirds of all people living with HIV are in sub-Saharan countries, where the epidemic has caused immense human suffering, drained already fragile health systems and had a negative impact on their economies.

The global response to Aids has been one of the most dedicated and forceful in the history of public health. HIV has been studied more intensively than any other virus and medical research has developed and steadily improved a range of treatments that can reduce HIV to undetectable levels, preserving the health and prolonging the lives of people living with the virus.

Unprecedented levels of funding through the United States’ President’s Emergency Plan for Aids Relief programme and the Global Fund to Fight Aids, Tuberculosis and Malaria have made antiretroviral therapy available to more than 6.5-million people in low- and middle-income countries, most of them in Africa.

Important developmentsThere have also been important recent developments in HIV prevention. Voluntary medical male circumcision and antiretroviral therapy to block mother-to-child transmission of HIV are proven strategies that are now being widely rolled out. The efficacy of new ARV-based prevention methods, including microbicides and treatment as prevention, is being demonstrated in research trials with several key populations.

But despite this effort and these achievements, the number of new HIV infections each year, though declining, is still unacceptably high.

An HIV vaccine is still desperately needed. There are two big questions that dominate the field of HIV vaccine research and development: Can it be done? And is it still needed?

HIV presents extraordinary barriers to vaccine development, more than any other virus. But advances in knowledge about the virus and the immune system, and in technologies for vaccine design and development, are producing steady results.

Most prominent was the announcement in 2009 that a large vaccine trial known as RV144 showed that people who received the test vaccine were 31% less likely to be infected with HIV than those who received the placebo. That proves that a vaccine is possible.

Effective vaccineIn addition, dramatic advances have been made in research into potent antibodies that are effective against a range of strains of HIV and in exploring systems, called vectors, for delivering a long-lasting, effective vaccine into the human system.

There is now consensus in the field that the discovery of an Aids vaccine is a matter of when, not if.

But will an HIV vaccine still be necessary when it is finally available? Last year, an important study confirmed what many earlier reports had indicated: people with HIV who receive antiretroviral treatment can bring their virus levels down to undetectable and are 96% less likely to transmit HIV to their partners.

With “treatment as prevention” in hand and a viable vaccine still out of reach, does it make sense to spend millions of dollars on antiretroviral vaccine research?

We know that an HIV vaccine would be the best tool for finally ending Aids.

We are still years from an Aids vaccine, but we now know that it can and must be done.

Michel Sidibe is executive director of the joint United Nations programme on HIV/Aids and under-secretary general of the United Nations, Chidi Nweneka is executive director of the African Aids Vaccine Partnership and Margaret McGlynn is chief executive officer of the International Aids Vaccine Initiative